Mr Maathai's Green Belt Movement planted 4.7 million trees, double the number of trees it had initially pledged, according to UNEP.

Experts says that trees help contain carbon that accumulates the heat-trapping gases blamed for climate change.

Although the figure could not be verified, it sends a powerful message ahead of the December 3-14 meeting in Bali of the UN Framework Convention on Climate Change (UNFCCC), a panel charting the path for negotiating pollution cuts to be implemented after 2012 when the Kyoto Protocol pledges run out.

"We called you to action almost exactly a year ago and you responded beyond our dreams," said Mr Maathai, who won the Nobel Peace prize for her campaign to plant tens of millions of trees to counter tree-loss and desertification in Africa.

"Now we must keep the pressure on and continue the good work for the planet," Mr Maathai said in the statement.